Airborne vehicle high frequency antenna

ABSTRACT

An aircraft antenna comprising an essentially balanced, folded dipole utilizing portions of the vehicle structure as the radiating elements. Conductors are spaced apart from, and faired into, the trailing edge of the main wings. High-frequency currents are distributed on the conductors and, via connections to the wing tips, the vehicle structure in a manner to provide essentially an omnidirectional radiation pattern in a predominantly horizontal polarization.

United States Patent [1 1 Martin et al.

[ Nov. 20, 1973 AIRBORNE VEHICLE HIGH FREQUENCY ANTENNA [75] Inventors: William R. Martin; Charles D.

Widell, both of Northridge, Calif.

[73] Assignee: Lockheed Aircraft Corporation,

Burbank, Calif.

22 Filed: June 30, 1972 21 Appl. No.: 267,987

[52} US. Cl. 343/708, 343/803 [51] Int. Cl. H0lq 1/28 [58] Field of Search 343/705, 708, 795,

[56] References Cited UNITED STATES PATENTS 2,242,200 5/1941 Woods 343/705 2,412,249 12/1946 Brown et a1. 343/705 2,659,004 11/1953 Lindenblad..... 343/705 3,005,986 10/1961 Reed 343/708 4 Primary ExaminerEli Lieberman Att0rneyGeorge C. Sullivan et al.

[57] ABSTRACT An aircraft antenna comprising an essentially balanced, folded dipole utilizing portions of the vehicle structure as the radiating elements. Conductors are spaced apart from, and faired into, the trailing edge of the main wings. High-frequency currents are distributed on the conductors and, via connections to the wing tips, the vehicle structure in a manner to provide essentially an omnidirectional radiation pattern in a predominantly horizontal polarization.

10 Claims, 4 Drawing; Figures PATENTEB Z 3,774. 220

SHEET 2 OF 2 HF TRANSMITTER RECEIVER HF ANTENNA 43 COUPLER HF BALUN m4l AIRBORNE VEHICLE HIGH FREQUENCY ANTENNA BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an electromagnetic-wave launching device and more particularly to an aircraft antenna which utilizes a portion of an aircrafts structure as a radiating element and which is compatible withthe limited dimensions of small airborne vehicles such as helicopters.

2. Prior Art Heretofore it has been proposed to employ portions of an aircrafts structure as radiating elements for the aircrafts radio communication system. One such prior method employs excitation of a notch located on one metal wing or tail surface. Other methods involve shunt excitation of one metal wing surface or isolation of a portion of the wing cap or tail cap. Probe antennas have also been installed in the aircraft nose, wing tip, and empennage tip. In these various installations, the primary consideration was to efficiently excite the airframe as an HF antenna. Most antenna configurations of the prior art have resulted in a compromise installation wherein the aerodynamic and structural considerations have prevented the maximum electrical effec tiveness to be realized. The non-uniformity of airframe or ground plane contour have added to the generally unsatisfactory performance of such antenna systems. As a consequence communications have not been consistent at all azimuthal angles.

Another disadvantage of these prior methods is that on small aircraft, such as helicopters, insufficient mechanical and electrical length is available for the radiating element. That is, for efficient performance of a transmitting antenna operating in the frequency spectrum from 2 megahertz to 30 megahertz requires physical dimensions which are incompatible with the aircrafts structure. Furthermore, prior devices attempting to overcome these shortcomings are particularly susceptible to damage when operating airborne vehicles from unimproved landing strips.

SUMMARY OF THE INVENTION The invention provides an aircraft antenna suitable for operation in the frequency range from 2 megahertz to 30 megahertz with continuous operational coverage out to 300 miles and which employs a radiating element which is aerodynamically faired into the secondary structural portion of the vehicle. In one embodiment, the antenna conductors are spaced near the vehicle fuselage as strakes and are faired into the trailing edge of the main wings. Dielectric materials are used to maintain spacing and insulate the conductor of the antenna from the vehicle structure.

The antenna of the present invention can be considered approximately as a horizontally-polarized electrically short type antenna. The radiation pattern in free space at the low-end of the HF band is essentially of the classic toroid shape. It will be recognized, by those versed in the art, that when an aircraft is operating at altitudes below 200 feet above the earth the elevation pattern will be strongly modified by the image ground reflection. This is due to the proximity of the ground in terms of electrical wavelength. As a consequence, higher elevation angles of the launched wave are required for short range transmission while lowangle radiation favors long-range communications.

In the present invention, radio frequency (RF) excitation currents are distributed along the antenna conductors and the vehicle structure in such a manner as to efficiently radiate electromagnetic energy. The radiation pattern is essentially omnidirectional and is predominantly horizontally polarized. The excitation of the aircrafts vertical stabilizer will yield radiation having a vertical polarization.

It is therefore an object of the invention to provide a novel and improved aircraft antenna having superior aerodynamic and radio-wave-pro pagating characteristics.

Another object of the invention is to provide a novel and improved folded-dipole antenna suitable for incorporation into the aerodynamic structure of a compound helicopter or the like.

A general object of the invention is to provide a novel and improved aircraft antenna which overcomes disadvantages of previous means and methods heretofore intended to accomplish generally similar purposes.

BRIEF DESCRIPTION OF Til-IE DRAWINGS FIG. 1 is a somewhat diagrammatic, bottom plan view of a compound helicopter incorporating a first embodiment of the invention.

FIG. 2 is a somewhat diagrammatic, bottom plan view of a compound helicopter incorporating a second embodiment of the invention.

FIG. 3 diagrammatically illustrates a radio communication system incorporating the antenna of FIG. 2.

FIG. 4 is a cross section of an aircraft wing incorporating an antenna constructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There is shown in FIG. 1 a first embodiment of the invention, as incorporated into a compound helicopter, which comprises radiating conductors 1 and 2 which are spaced near the vehicle fuselage 3 as strakes and aerodynamically faired into the trailing edge of the fixed wings 4 and 5. Spacing between conductors 1 and 2 and the vehicle structure (3) is maintained by strips of dielectric material 7 and 8 such as laminated glass fiber or the like. Conductors l and 2 are grounded to the vehicle wing structure at points I fant i High-frequency RF currents are distributed via conductors l and 2, and the vehicle: structure (3), in a manner to provide essentially an omnidirectional radiation pattern in a predominantly horizontal polarization at an angle with the earths surface that directs the energy high above the horizon. Such directionality makes use of the reflective properties of the E and F ionospheric layers which redirect the RF energy back towards the earth. Circulating currents in the vehicle structure (3) will result in RF excitation of the vehicles vertical stabilizer l3 and will radiate vertically polarized energy in an increasing amount as the operating frequency approaches 30 megahertz. The antenna conductors 1 and f emer ge from the fusel ageijlat feed points 14 and 15. Feed points 14 and 15 may be energized from any suitable and well-known radio transmitter, as will be readily understood by those versed in the art.

There is shown in FIG. 2 a second embodiment of the invention as incorporated into a compound helicopter. The antenna radiating elements 16 and 17 originate at feed point 18, which point is insulated from fuselage 19 and sponsons 21 and 22, and are attached to the metallic conductors 23 and 24 respectively. Conductor 23 is imbedded in the trailing edge of the right wing 2 The conductor 23 imbedded in the trailing edge of wing 25 is separated and insulated from the metallic structure portion of the wing (25) along the entire trailing edge span by means of an interposed dielectric separator 29, except at the outboard wing tip 27. At this point (27), the conductor 23 is electrically connected to the metallic structure of the wing 25. Details of the wing conductor 23 and dielectric separator 29 will be described more fully hereinafter in connection with FIG. 4.

A similar configuration exists for the left side of the aircraft. Specifically, radiating element 17 extends outwardly from feed point 18 and is insulated from fuselage I9 and sponson 22. Thence it extends along the trailing edge of left wing 26 as conductor 24 which is spaced apart from the metal portion of wing 26 by means of dielectric separator 31. The outboard end of conductor 24 is electrically connected to the metal wing 26 at wing tip 28.

The configuration of the conductors (23 and 24) is the same on the left and right sides of the aircraft; therefore, the installation comprises an electrically balanced antenna system, which includes the conductors 23-24, wings 25-26, sponsons 21-22, and fuselage 19, all of which takes the form of a folded dipole antenna.

There is shown in FIG. 3 a high-frequency (I-IF) balun assembly used in conjunction with the invention to transform a single-conductor, unbalanced transmission line to a two-wire balanced transmission line, which is then attached to the antenna feed point 39. Because of the balanced configuration of the antenna, a simple balun may be inserted to properly match the antenna to an antenna coupler and transmitter. The impedance transformation range presented by the balun will be within the tuning range of a standard HF coupler. The antenna comprises conductors 32 and 33 which run parallel to and are electrically spaced apart from the conductive trailing edge of wings 34 and 35, respectively. Conductor 32 is electrically connected to the conductive portion of the wing at point 36. In a like manner, conductor 33 is connected to wing 35 at point 37. This arrangement, in combination with the conductive path between the wings 34 and 35 via fuselage 38 functions essentially as a folded dipole antenna having a horizontal radiation pattern directed principally along the longitudinal axis of the aircraft. The feed point (39) terminals are insulated from the skin and metallic structure of the vehicle fuselage 38. Highfrequency balun 41, of any suitable and well-known construction, is connected between feed point 39 and unbalanced transmission line 42. The RF antenna coupler 43 is connected between line 42 and singleconductor unbalanced, transmission line 44. The highfrequency transmitter/receiver 45 is connected to the HF coupler 43 through transmission line 44. The radiating element (32-33) is mechanically and electrically attached to the wing tip spar (36-37). The metallic structure (46) is carefully bonded to provide uniform RF current distribution. The inboard section 49 of the radiating element 32-33 is mounted across the lower sponson and fuselage area 38.

Since there are no movable surfaces on the wing trailing edge of a compound helicopter, the full length of the structure may be used as an element of the antenna. Radiation pattern symmetry in azimuth is maintained by the excitation of both wing sections. As described previously, a folded dipole configuration is obtained by connecting the outboard end of the radiating elements to the wing tip spars, thereby providing increased bandwidth properties. The feed point is preferably located at the bottom centerline of the fuselage with the radiating elements extending outwardly to the wing trailing edge.

In the embodiments shown, wherein the antennae are mounted on a compound helicopter, at 2 megahertz the aircraft is approximately 0.1 wavelength long, while at 30 megahertz it is approximately 1.5 wavelengths long. Therefore, the radiation characteristics of any I-IF antenna installed on a vehicle this small are determined by the distribution of RF currents on the surface of the fuselage, wings, main rotor, and empennage. The distinction between the subject antenna and existing HF airborne antennas is the omnidirectional coverage provided throughout the operating range of frequencies whereas omnidirectional coverage for existing antennas have been limited to the upper frequency band in vertical polarization, usually obtained from excitation of vertical structures by such devices as long wires, notches, probes, helices, or isolated sections. As can be seen, the construction of the present invention provides a shunt forward wing section, balanced when both wing sections are used, to support the direct paths of the HF antenna current. In this manner, a completely flushmounted antenna assembly is provided without sacrificing primary structure loading, aerodynamic drag, or the safety of ground handling personnel. Aside from the antenna coupler, the system has no moving parts, thereby assuring maximum reliability.

Referring to FIG. 4 there is shown a cross section of an aricraft wing such as that indicated at 34 in FIG. 3. The wing comprises an electrically conductive structure 46 having a continuous outer metal skin. A hollow metal tubular element 47 of triangular cross section comprises the antenna conductor. The antenna conductor (4'7) is spaced apart from the conductive wing structure 46 and supported therefrom by dielectric strips 48 and 49 which are faired into the outer metal skin of the wing 46. In a preferred construction, dielectric strips 48-49 comprise laminated glass fiber sheets. An air space, indicated at 51, separates the forward side of conductor 47 and rearward side of wing member 46.

Summarizing, from the foregoing it will be seen that there is provided by the present invention a novel and improved airborne antenna wherein the aerodynamic performance of the aircraft is not impeded because of increased drag. Since the RF energy is launched at a high angle to the horizon, there is an improved continuous coverage throughout the desired range. Prior antennas left operational gaps in the desired range. Since certain changes may be made in the above-described embodiments without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a vehicle having an electrically conductive exterior surface, an antenna comprising:

means defining first and second conductive paths extending outwardly from a centrally disposed feed point;

dielectric means extending along and supporting said first and second conductive path defining means in substantially parallel spaced-apart relationship with respect to said conductive exterior surface; and

means for electrically connecting the extremities of said first and second conductive path defining means to said conductive exterior surface, thereby completing a folded-dipole antenna circuit path therethrough.

2. Antenna apparatus as defined in claim 1 wherein said conductive exterior surface comprises the trailing edges of a pair of electrically bonded wings of an aircraft.

3. Antenna apparatus as defined in claim 1 wherein said dielectric means comprises an elongate nonconductive strip secured to said exterior surface and wherein said first and second conductive path defining means are imbedded within said strip.

4. Antenna apparatus as defined in claim 1 wherein each of said first and second path defining means has an L-shape so as to extend in a first direction from said feed point over a portion of the overall length thereof, and thereafter extend in a generally orthogonal direction over the remaining portion thereof to said connecting means.

5. Antenna apparatus as defined in claim 1 wherein said vehicle comprises an aircraft, wherein each of said first and second path defining means has an L-shape so as to extend in a first direction from said feed point over a portion of the overall length thereof and thereafter extend in a generally orthogonal direction over the i means connecting the ends of said radiating elements, opposite said feed point, to said wing.

7. An antenna as defined in claim 6 wherein each of said radiating elements comprises:

a first strake portion mounted in substantially parallel, spaced-apart relationship with respect to the major axis of said aircraft; and

a second trailing edge portion extending outwardly from said strake portion in a substantially transverse relationship with respect to said major axis.

8. An antenna as defined in claim 6 wherein said feed point is disposed at the centerline of said aircraft and each of said radiating elements comprises a first insulated conductor portion extending from said feed point to a location intermediate the ends of said wing, and a second radiating portion imbedded within a corresponding one of said dielectric spacers.

9. An antenna as defined in claim 6 wherein said dielectric spacers each comprise:

a substantially V-shaped dielectric strip faired into the trailing edge of said wing, and enclosing a corresponding one of said radiating elements.

10. An antenna as defined in claim 6 including: balun means connected to said feed point. 

1. In a vehicle having an electrically conductive exterior surface, an antenna comprising: means defining first and second conductive paths extending outwardly from a centrally disposed feed point; dielectric means extending along and supporting said first and second conductive path defining means in substantially parallel spaced-apart relationship with respect to said conductive exterior surface; and means for electrically connecting the extremities of said first and second conductive path defining means to said conductive exterior surface, thereby completing a folded-dipole antenna circuit path therethrough.
 2. Antenna apparatus as defined in claim 1 wherein said conductive exterior surface comprises the trailing edges of a pair of electrically bonded wings of an aircraft.
 3. Antenna apparatus as defined in claim 1 wherein said dielectric means comprises an elongate non-conductive strip secured to said exterior surface and wherein said first and second conductive path defining means are imbedded within said strip.
 4. Antenna apparatus as defined in claim 1 wherein each of said first and second path defining means has an L-shape so as to extend in a first direction from said feed point over a portion of the overall length thereof, and thereafter extend in a generally orthogonal direction over the remaining portion thereof to said connecting means.
 5. Antenna apparatus as defined in claim 1 wherein said vehicle comprises an aircraft, wherein each of said first and second path defining means has an L-shape so as to extend in a first direction from said feed point over a pOrtion of the overall length thereof and thereafter extend in a generally orthogonal direction over the remaining portion thereof to said connecting means, and wherein said conductive exterior surface comprises the trailing edges of a pair of electrically interconnected wings of said aircraft.
 6. An antenna adapted to be mounted at the wing trailing edge of an aircraft, having a conductive wing, comprising: a pair of radiating elements having an intermediate feed point; a pair of dielectric spacers each supporting a corresponding one of said radiating elements in spaced apart relationship with respect to said trailing edge; and, means connecting the ends of said radiating elements, opposite said feed point, to said wing.
 7. An antenna as defined in claim 6 wherein each of said radiating elements comprises: a first strake portion mounted in substantially parallel, spaced-apart relationship with respect to the major axis of said aircraft; and a second trailing edge portion extending outwardly from said strake portion in a substantially transverse relationship with respect to said major axis.
 8. An antenna as defined in claim 6 wherein said feed point is disposed at the centerline of said aircraft and each of said radiating elements comprises a first insulated conductor portion extending from said feed point to a location intermediate the ends of said wing, and a second radiating portion imbedded within a corresponding one of said dielectric spacers.
 9. An antenna as defined in claim 6 wherein said dielectric spacers each comprise: a substantially V-shaped dielectric strip faired into the trailing edge of said wing, and enclosing a corresponding one of said radiating elements.
 10. An antenna as defined in claim 6 including: balun means connected to said feed point. 